The present invention relates generally to radar systems, and more particularly to a method for measuring the doppler shift of frequency of an echo due to relative movement of a target and a point of reception of the echo, and a system embodying the method.
The fact that the relative movement of a target and a radar system can be determined by transmitting a beam of electromagnetic radiation of a predetermined frequency, usually in the microwave region, toward the target and measuring the deviations in frequency of an echo from the predetermined frequency is well known in the art. The doppler shift of frequency is an extreme fraction of the transmitted frequency which is a function of the angle made between the direction of propagation of the beam and the direction of movement of the target. Where the beam diverges as it propagates toward the target, the angle it makes with the direction of propagation will vary with time as the target advances. The signal intensity of a received echo arising at the detectable fringe areas of the diverging beam is much weaker than in the intermediate portion of the beam, and thus the doppler shift frequency will violently fluctuate due to possible interfering noise which occurs as the target is about to enter and leaves the fringe areas. The use of a larger diameter antenna may provide a narrow beam of strong intensity to obviate such difficulty only with the consequential increased cost. In addition, it is difficult to compensate for the varying angle for measurement of doppler shift and consequently an error will be introduced into the measurement if no appropriate compensation is made.